Scintillator single crystals are used for radiation detectors that detect gamma rays, X rays, alpha rays, neutron rays, or the like. Such radiation detectors are widely used for medical imaging apparatuses (imaging apparatuses) such as positron emission tomography (PET) apparatuses and X-ray CT apparatuses, various radiation measuring apparatuses in the field of high-energy physics, resource exploration apparatuses (resource exploration such as oil well logging), or the like.
A radiation detector generally includes a scintillator that absorbs gamma rays, X rays, alpha rays, or neutron rays and converts the rays into scintillation light and a photodetector such as a light-receiving element that receives the scintillation light and converts the scintillation light into an electric signal or the like. In high-energy physics or positron emission tomographic (PET) imaging systems, for example, images are created based on interaction between the scintillator and radiation generated through radioactive decay. In positron emission tomographic (PET) imaging systems, gamma rays generated through interaction between positrons and corresponding electrons within a subject enter the scintillator and are converted into photons, which can be detected by a photodetector. The photons emitted from the scintillator can be detected using photodiodes (PDs), silicon photomultipliers (Si-PMs), photomultiplier tubes (PMTs), or other photodetectors.
The PMT has a high quantum yield (efficiency about converting photons into electrons (a current signal)) in the wavelength range of about 400 nm and is used in combination with a scintillator having a peak emission wavelength of mainly about 400 nm. For a scintillator array, in which scintillators are arranged in an array, a position sensitive PMT (PS-PMT) or the like is used in combination. With this configuration, it can be determined which pixel of the scintillator array has detected a photon from gravity computation.
Semiconductor photodetectors such as photodiodes (PDs), avalanche photodiodes (APDs), and silicon photomultipliers (Si-PMs) have wide applications in radiation detectors and imaging devices in particular. Various semiconductor photodetectors are known. PDs and Si-PMs formed of silicon semiconductors, for example, have a quantum efficiency of 50% or more in the wavelength band from 350 nm to 900 nm and have higher quantum efficiency than PMTs, the quantum efficiency of which is 45% at most. In the above wavelength band, the sensitivity is higher in the wavelength band from 500 nm to 700 nm and peaks at around 600 nm, in which the quantum efficiency is about 80%. Given these circumstances, these semiconductor photodetectors are used in combination with scintillators having a peak emission wavelength in the range from 350 nm to 900 nm, centering at around 600 nm. Similarly to PMTs, also for PDs, APDs, and Si-PMs, there exist PD arrays having position detection sensitivity, position sensitive avalanche photodiodes (PSAPDs), and Si-PM arrays. These elements can also determine which pixel of the scintillator array has detected a photon. Furthermore, even for short-wavelength light emission scintillators of 350 nm or less, scintillation light is converted into light with a wavelength range in which silicon semiconductors have sensitivity by, for example, using Si-PMs for short wavelength or wavelength conversion elements, thereby achieving radiation detectors that perform reading by silicon semiconductors.
The scintillator adapted for these radiation detectors is desired to have high density and a large atomic number (a high photoelectric absorption ratio) in view of detection efficiency, a large light emission amount in view of high energy resolution, and a short fluorescence lifetime (fluorescence decay time) in view of the necessity of high-speed response. In addition, in recent years' systems, many scintillators are required to be densely arranged in a long, narrow shape (about 5 mm×30 mm for PET, for example) for the purpose of achieving a multilayered structure and increased resolution, and important selection factors include handleability, processability, capability of producing large crystals, and a price. It is also important that the light emission wavelength of the scintillators matches the wavelength range with high detection sensitivity of the photodetectors.
Examples of scintillators currently suitable for the application to various radiation detectors include a scintillator Ce:Gd2Si2O7 having a pyrochlore type structure. The scintillator has the advantages of chemical stability, the absence of deliquescence, and being large in light emission amount. A scintillator having the pyrochlore type structure using light emission from the 4f5d level of Ce3+ described in Non Patent Literature 1, for example, has a short fluorescence lifetime of about 80 ns or less and also has a large light emission amount. On the other hand, however, as described in Non Patent Literature 1, the scintillator has the problem that because of being a peritectic composition on the phase diagram, single crystal growth from a melt is impossible, and it is difficult to obtain a large transparent body.
Scintillators having the pyrochlore type structure described in Patent Literature 1 and Patent Literature 2 and Non Patent Literature 2 are attempted to stabilize the structure by substituting Ce for rare-earth-element sites. With this stabilization, this crystal can be produced as a large single crystal by melt growth methods such as the floating zone method, the Czochralski method, the micro pulling-down method, and the Bridgman method. However, increased Ce at the sites of the rare earth element leads to the problem (concentration quenching) that the light emission amount drastically decreases.
Patent Literature 3 and Patent Literature 4 and Non Patent Literature 3 attempt to stabilize the structure by substituting one or more elements selected from Y, Yb, Sc, La, and Lu for the rare-earth-element (especially La) sites. With this stabilization, it was expected that this crystal would be able to be produced as a large single crystal by melt growth methods such as the floating zone method, the Czochralski method, the micro pulling-down method, and the Bridgman method. Although the attempt produces some effects, in (R,R′)2Si2O7, a stoichiometric composition of (R+R′):Si:O=2:2:7 is not a congruent melting composition, impurities are produced and captured as a heterophase when a bulk single crystal is produced, which causes cracks and defects, and thus the problem that a large-diameter bulk single crystal cannot be obtained with high yield has been newly found.
Furthermore, a nonstoichiometric composition shifts the balance of electric charges, which may cause problems such as the occurrence of strain in the crystal and influence on a fluorescence lifetime.